1. Field of the Invention
The invention disclosed herein relates to oil and gas exploration and, in particular, to a circuit design for implementing a resistivity measuring device.
2. Description of the Related Art
A variety of technologies is used for oil and gas exploration. One such technology evaluates electrical properties of sub-surface materials and formations. Instruments implementing such technology are commonly referred to as “resistivity” instruments.
Typically, resistivity instruments are either lowered into a borehole that has previously been drilled into the earth, or incorporated into a drilling apparatus. Often, some supply of drilling mud is introduced into the borehole. The drilling mud, provided to tamp down pressure from below, may be oil based mud or non-oil (e.g., water) based mud. Each type of mud may present particular concerns to users implementing resistivity measuring technologies due to varying natures of conductivity.
The various electrode designs presently in use in resistivity instruments involve a variety of configurations. Each of the various configurations has been developed to address some deficiency of prior designs. One continuing problem is the incorporation of a systematic error in measurements of apparent resistivity. The error can be large if the resistivity of the mud, Rm, is very low.
An exemplary instrument for making resistivity measurements is available from Baker Hughes, Incorporated of Houston, Tex. The instrument, referred to as an “Earth Imager,” has provided for a variety of resistivity images.
With regard to the exemplary instrument, reference may be had to FIG. 1. In FIG. 1, there is shown a depiction of the prior art instrument 21 for performing resistivity imaging. In this example, the instrument 21 is disposed within a wellbore 11 (also referred to as a “borehole”). The instrument 21 includes pads 3 mounted on articulating arms 2. The articulated pads 3 are typically pressed up against a wall of the wellbore 11 and make firm contact therewith. Current I flows from at least one transmitter electrode 6 on the pad 3 to at least one return electrode 4. The return electrode 4 is electrically separated from each transmitter electrode 6 by an insulator 5. The current I is typically alternating current (AC) (and, therefore it is recognized that the current I actually flows in two directions as polarity changes).
In a variety of embodiments, the transmitter electrode 6 is accompanied by at least one guard electrode 7. The guard electrode 7 is provided to drive measuring current into the formation 10, thus preventing or limiting measuring current from flowing along the wellbore 11. Unfortunately, current from the guard electrode 7 may also enter the measuring current, and cause systematic measurement error. As discussed above, variations of this instrument 21 and others may be used in drilling apparatus to perform measurements while drilling.
Therefore, what are needed are methods and apparatus relating to a resistivity instrument that reduces systematic error in results for measurements of apparent resistivity. Preferably, the methods and apparatus include capabilities to correct systematic errors arising during measurement processes.